1. Field of the Invention
This invention relates to polyolefin/filler composite materials, and their preparation. The invention relates in particular to polyolefin/filler composite materials in which the filler comprises hollow microspheres. Such materials are known as polyolefin "syntactic foams".
2. Description of the Prior Art
The main proposed field of use of the materials of the present invention is thermal insulation, in particular insulation of underwater flowlines. There are onerous requirements on an insulating material for an underwater flowline. Desirably, the material should:
have a low Melt Flow Index so that it can be extruded in a pipe extrusion process PA1 be resistant to microsphere breakage during the preparation of the material and subsequent application to a pipe PA1 permit substantial tensile elongation, to enable a flowline to be installed by any installation method including reel barging PA1 exhibit low creep over a long period, perhaps a service life of 30 years, during which hot oil may be conveyed within the flowline PA1 have excellent mechanical properties. PA1 treating at least some of the microspheres to be incorporated in the polyolefin with a compound which acts as a chain-scission agent for the polyolefin; PA1 providing an even distribution of said microspheres in said polyolefin in its melt phase, and PA1 forming the composite material into a desired shape and cooling.
Polyolefin/filler composite materials in which the filler comprises "glass bubbles" are described in Research Disclosure 253015. This article discloses that it is possible to compound "glass bubbles" with inter alia polypropylene using extrusion equipment with less than 5% by weight breakage, to produce low-density composite materials. Proposed uses thereof are extruded parts for buoyancy and thermal insulation and large automotive moldings.
DE 2445075 describes a film coating in which microspheres are coated with a small amount of polymer, then applied to a support and heated. The polymer may comprise a peroxide e.g. 3-10 parts per 100 parts polymer. The use of polypropylene or polybutylenes is not described or proposed.
WO87/0478 relates to a "combined weight- and insulation casing for subsea pipelines". An extrusion technique is employed. Polypropylene and polyvinyl chloride are said to be especially suitable and it is said that these can be foamed by means of a chemical foaming agent or by addition of microspheres of plastic or glass to the melt. However, only chemical foaming is exemplified.
When it was attempted, during experimentation, to incorporate microspheres into a polyolefin to make a composite material suitable for rigorous applications such as flowline insulation, it was found to be impossible to do so adequately on the basis of prior art techniques. It was found that, as indicated in the prior art, it was possible to incorporate microspheres into polypropylene and to extrude the composite material without breakage of the microspheres, but only using base polypropylenes of high Melt Flow Index (MFI), which were of low molecular weight and low viscosity in the melt phase. The resultant composite materials generally had Melt Flow Indexes which were too high for application by the "pipe extrusion" method and/or have unsuitable mechanical properties for flowline insulation. Furthermore, it is known that the creep resistance of materials of high Melt Flow Index is poor. When it was attempted to incorporate microspheres in a base polypropylene of low Melt Flow Index, it was found that a substantial proportion of the microspheres were broken during the compounding or subsequent extrusion, so that the insulation properties were inadequate.
Thus the proposal in the Research Disclosure that "glass bubbles" can be compounded with polypropylene and the resultant composite material extruded would only appear to hold good for what might be termed non-rigorous applications, whilst the unsupported suggestion in WO 87/0478 that microspheres can be employed in extruded pipeline casings appears not to be achievable, at least on the basis of available technology.
The present invention aims to provide a method whereby microspheres may be incorporated in a polyolefin matrix to form a composite material which can be employed in rigorous application areas, for example as an insulating casing for an underwater flowline, and wherein the properties of the composite material may be closely controlled.